In general, transformers are classified into indoor and outdoor transformers according to places where they are installed or into dry and oil transformers according to the use of insulating oil.
As the indoor transformer, a fire retardant dry transformer is mainly used in view of the prevention of disaster in a place where it is installed. Examples of the indoor transformer may include an impregnated transformer (e.g., a mold transformer) and an unpotted transformer (e.g., a Vacuum Pressure Impregnation (VPI) transformer). In case of a 22.9 kV dry transformer, a technique of placing a coil into a mold and performing vacuum molding by using epoxy resin and the like has been conventionally used in order to maintain insulation to a high voltage of the first side. Recently, the coil itself is vacuum/pressure varnished instead of being molded with epoxy resin.
However, in a case where the dry transformer is installed outdoor, external influence such as rain, wind, moisture and dust may disable the insulation of the coil thereby causing a serious problem. Accordingly, the dry transformers are generally limited to indoor applications. For outdoor applications, oil transformers are being adopted, in which insulating oil is used to insulate a transformer body composed of coils and a core.
Such an oil transformer is fabricated by placing a transformer body into a housing and filling insulating oil into the housing for the purpose of coil insulation and cooling. A radiator may also be provided to the housing to facilitate the circulation of the insulating oil owing to heat as well as to expand heat radiating area, thereby improving cooling efficiency.
For the insulating oil filled in the conventional transformer for power distribution, silicone oil or mineral oil has generally been used. However, the silicone or mineral oil disadvantageously has a low permittivity. Furthermore, the mineral oil has a low burning point of about 140° C., which involves a risk of explosion. To overcome such a problem, an approach of using the silicone oil having a burning point of 300° C. was proposed. However, the silicone oil does not circulate well inside the transformer owing to rather poor viscosity, thereby degrading cooling efficiency. Furthermore, since the insulating oil is a type of industrial oil counter to environmental requirements, it may disadvantageously cause environmental pollution when leaked or discarded from the transformer. In particular, when used for a long term, the insulating oil produces polychlorinated biphenyl (PCB) that has a fatal effect to the human body.
Accordingly, there have been urgent demands for a novel approach in the art, by which the dry transformer that does not use the insulating oil but its use has generally been restricted to indoor applications can be used for outdoor applications in hostile environments.